Session Initiation Protocol (SIP) is an application-layer control (i.e., signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions may include Internet-based telephone calls, multimedia distribution, multimedia conferences, instant messaging conferences, interactive voice response (IVR), automated and manual operator services, automatic call distribution, call routing, etc. SIP invitations or SIP INVITE requests may be used to create sessions and may carry session descriptions that allow participants to agree on a set of compatible media types. SIP may use proxy servers to help route requests to a user's current location, authenticate and authorize users for services, implement provider call-routing policies, and/or provide other features to users. SIP may also provide a registration function that allows users to upload their current locations for use by proxy servers.
Integrated Services Digital Network (ISDN) User Part (ISUP) is a part of the Signaling System #7 (SS7) that is used to set up telephone calls in the Public Switched Telephone Network (PSTN). The field of SIP-to-ISUP and ISUP-to-SIP communications uses a defined standard mapping of ISUP messages to SIP messages, and defines which parameters are copied and how they are copied. For example, an ISUP-to-SIP gateway may implement the standard mapping. However, routing of calls from a PSTN (e.g., an ISUP-based network) to a SIP network does not expose special capabilities of the SIP network to the PSTN. For example, proprietary SIP headers, which are not defined in the standard mapping, will not be populated.
One feature missing from the standard mapping is support for a SIP REFER method (e.g., special call transfer capabilities supported by SIP, as described in the Internet Engineering Task Force (IETF) document RFC 3515). For example, an ISUP-to-SIP gateway is unable to map the SIP REFER method, and the PSTN provides no such functionality.